The present invention relates to alloys for use in the removal of bismuth from lead by the Kroll-Betterton process, or for use in similar lead refining processes which require the use of alkaline-earth metals.
In the Kroll-Betterton process, alkaline earth metals are added to the lead melt in order to react with bismuth impurities present therein. One or more alkaline earth metals, usually magnesium and calcium, are added in either a continuous or batch fashion to the unrefined lead. The preferred temperature range for making the addition is between 380.degree. C. to 500.degree. C. Below this temperature range, the reaction is sluggish while above this range excessive oxidation of reactive alkaline earth metals, particularly calcium, occurs. Oxidation gives rise to bright flaring, excessive fume generation and an overall loss of reagent leading to lower reagent recoveries, excessive processing costs, unpredictable final bismuth levels and environmental concerns.
Furthermore, the addition of calcium metal to the lead bath is often accompanied by an increase in the bulk temperature of the lead either due to an exothermic release of heat during the reaction and/or the heat generated by the oxidation of calcium metal. This increase in bath temperature may result in additional calcium oxidation as well as lengthening the overall processing time since the melt must be cooled to just above its liquidus point prior to removing the bismuth rich dross
Another disadvantage of calcium metal is that it is highly reactive with atmospheric oxygen and humidity. Hence, calcium metal must be packaged, shipped and stored in such a way as to eliminate contact with air and moisture. Excessive contact with water will result in heat and hydrogen evolution which can cause fire and explosion. Mild contamination of the calcium prior to the lead treatment will result in lower than expected reagent recoveries and unpredictable final bismuth levels.
After the lead has been treated with the alkaline metals, the melt is then cooled to a temperature near its liquidus point which causes the resulting alkaline-earth bismuth compounds to float up as a solid dross which may be skimmed from the surface of the melt to thus purify the melt.
Most commercial debismuthizing processes utilize a heterogeneous mixture of magnesium and calcium metals. In the present invention, debismuthizing is carried out with an alloy substantially comprised of magnesium and calcium with the ratio of magnesium to calcium on a weight basis being between about 1.2:1 to about 5.2:1 and, in a preferred embodiment of the invention, between about 1.85:1 to about 3.0:1.
The concept of substituting alloys for metallic magnesium and calcium was initially suggested by Betterton in 1930, as described in U.S. Pat. No. 1,853,540, who tested alloys comprised of magnesium and lead and calcium, magnesium and lead.
T.R.A. Davey "The Physical Chemistry of Lead Refining", Lead-Zinc-Tin 1980, edited by J. M. Cigan et al., Metallurgical Society of AIME, p.477, mentions the use of a 5% calcium-lead alloy while Kirk-Othmer "Lead", Encyclopedia of Chemical Technology, Vol. 8, The Interscience Encyclopedia Inc., New York, 1952, refers to a 3% calcium-lead alloy. In all of these cases, lead is the principal alloying constituent and is present to lower the melting point of the reagent, thus promoting dissolution of magnesium, and in particular calcium, both of which have melting points substantially higher than the lead bath temperature.
In U.S. Pat. No. 2,129,445, Rehns mentions that lead can be debismuthized by floating a calcium-magnesium alloy on the surface of a mechanically stirred lead bath. The disclosed alloy contains 79.4% magnesium and 20.6% calcium by weight. Rehns specifically points out that when using a calcium-magnesium alloy of the cited composition, it is necessary that the lead bath be raised to a higher temperature, namely 593.degree. C.
Reference to a binary magnesium-calcium phase diagram (FIG. 1) shows that the addition of calcium to magnesium will initially lower the melting point of the alloy compared to metallic magnesium. However, once the alloy exceeds 16.2% calcium (i.e., a Mg to Ca ratio of 5.17), its melting point begins to rise due to an increasing concentration in the eutectic of the highly stable intermetallic compound, Mg.sub.2 Ca. This stable compound has a melting point of 715.degree. C. which is about 200.degree.-300.degree. C. above commercial debismuthizing temperatures.
The same phase diagram also shows that the 79.4% magnesium, 20.6% calcium alloy suggested by Rehns begins to melt at 516.5.degree. C. and is fully molten by about 575.degree. C. By specifying a lead bath temperature of 593.degree. C., Rehns ensures that this alloy will be fully molten and hence its dissolution and the resulting reagent recovery will not be impeded by the presence of any unmelted, highly stable Mg.sub.2 Ca intermetallic compound.
Kroll-Betterton type debismuthizing processes usually operate in the 380.degree. C. to 500.degree. C. range. Rehn's specified lead bath temperature of 593.degree. C. is thus substantially higher than reported commercial debismuthizing practices.
In the present invention, magnesium-calcium alloys with magnesium to calcium ratios on a weight basis between about 1.2:1 and about 5.2:1, and preferably between about 1.85:1 and about 30:1, are added to lead in the commercial temperature range, that is between 380.degree. C. to 500.degree. C. As indicated by the relevant phase diagram, all of these alloys have melting points in excess of 516.5.degree. C. and, in the range of the preferred embodiment, the alloys do not fully melt until temperatures range between 610.degree. C. to 685.degree. C., which temperatures are substantially above the temperature of the lead bath. Contrary to the teachings of the Rehns patent, which ensures that the alloy is completely melted by specifying a higher process temperature of 593.degree. C., in the present invention the alloys do not completely melt and hence the reaction must proceed by dissolving (not melting) a solid alloy into liquid lead.
According to the eutectic composition of such alloys, this solid phase is essentially the stable, high melting point Mg.sub.2 Ca intermetallic compound. Hence, the present invention differs from that of Rehns since the mechanism of introducing the reagent into the lead is considerably different.
In Rehns, the rate of reaction depends only on how fast the alloy melts which in turn depends on the rate of heat transfer from the bath to the reagent. Once melted, any Mg.sub.2 Ca compound present in the alloy is completely dissociated and hence available for debismuthizing.
In the present invention, the rate at which the solid Mg.sub.2 Ca phase in the alloys eutectic dissolves into the liquid lead depends on thermodynamic and kinetic considerations which are related to the chemical stability of Mg.sub.2 Ca relative to magnesium-calcium-bismuth compounds which form during debismuthizing. The rate of dissolution and hence the degree of dissociation of Mg.sub.2 Ca in the alloy has significant commercial significance as it will determine processing time and reagent recoveries.
French Patent Application No. 81 19673 assigned to Extramet (Publication No. 25614 786, Apr. 22, 1983) discloses a process for debismuthizing lead by using a mixture of two types of alloy granules. The first type of granule comprises a calcium-magnesium alloy near the calcium-rich eutectic point (approximately 82 weight % calcium) and the second alloy comprises a magnesium-calcium alloy near the magnesium-rich eutectic point (approximately 16.2 weight % calcium). These two types of granules are mixed together in the appropriate amounts to give the ratio of the metals for the best result and are injected into the lead melt to react with bismuth present therein. The composition of the individual alloys is chosen to be near the eutectic points so that they have relatively lower melting points compared to pure magnesium and calcium metals. It is claimed that this speeds up the rate of the reaction at a given processing temperature. The mixture is injected into the lead bath with an inert gas. The temperature of the lead bath is maintained high enough to melt and not simply dissolve the granules.
This heterogeneous mixture of magnesium-rich calcium-rich alloy granules is still susceptible to poor reagent recovery because the calcium-rich alloy granules will behave in much the same way as pure calcium metal. Because of the composition of calcium-rich eutectic alloy granules, the eutectic may contain up to almost 2/3 of finely divided calcium metal with the remainder being the Mg.sub.2 Ca intermetallic compound. The high proportion of calcium metal in the eutectic causes the calcium-rich alloy granules to react with atmospheric oxygen and humidity in much the same way as calcium metal. Tests with ingots cast at the calcium-rich eutectic composition have shown that this alloy reacts with atmospheric oxygen and humidity and, hence, is not stable in air.
Because of the reactive nature of the calcium-rich granules, the heterogeneous granule mixture of magnesium-rich granules and calcium-rich granules must be packaged under dry, inert gas in a similar fashion to calcium metal. Contamination of the calcium-rich granules with oxygen or moisture prior to treatment will result in lower reagent recoveries and unpredictable final bismuth level. The calcium-rich granules are also susceptible to oxidation during treatment with the lead in much the same way as calcium metal, especially if they float to the surface before they have completely reacted due to the large differences in density between lead and calcium. The injection of the granules into the lead bath with an inert gas carrier adds additional turbulence to the melt, increasing the amount of oxidation and emissions from the lead bath.
In the present invention, the difficulties associated with the use of calcium metal or granular mixtures containing calcium-rich alloy granules are avoided by using a single magnesium-calcium alloy of the desired composition. In this invention, the alloy is primarily composed of magnesium and calcium but may contain one or more minor amounts of other alloying elements.